Electrical properties of the insulator silicon nitride ceramics may be improved by addition of electrical conductive parts. The conductive carbon parts were mixed with the base ceramic matrix to form a percolation network. Electrical current can flow through the ceramic by using the connected carbon channels. In air atmosphere however, the carbon can oxidize and burn out. Heat treatments were performed to observe the carbon degradation in composites in atmosphere. As resulted, the carbon exhaust started at 400°C from surface and finished above 750°C. Electrical measurements showed the conductor-insulator transformation. Thermo-gravimetric measurements suggested that some carbon inclusion still remained in isolated closed porosities.